1. Field of the Invention
The invention concerns a sheet or film for use in the manufacture of securities documents, which is imprintable with high quality print, as well as being very resistant to the effects of circulation.
More particularly, the invention concerns paper sheets for banknotes or other valuable securities that can be imprinted by offset printing and/or by intaglio printing. Moreover, it relates to a composition for surface treatment of or for impregnating the face of a sheet in such manner as to simultaneously confer to it the properties of good imprintability and resistance to the effects of circulation.
2. Description of the Related Art
It is known that securities documents such as the paper for banknotes or checks or any other valuable securities comprise physical or chemical identification means and/or forgery indicators.
As a rule, it is known to use as chemical means compositions which react to the forgery agents presently used by forgers. These forgery indicator means react, for example, with acids (hydrochloric, citric, acetic, sulfuric etc.), with bases (especially soda), with oxidizers (javelle water), with reducing agents and with solvents.
It is also known to utilize physical identifiers which most often are the following:
The lack of brighteners fluorescing in blue-violet when exposed to ultra-violet light which presently are used in ordinary papers to increase their whiteness. PA1 The exact presence of elements (for example, threads, fibers, disks etc.) which are fluorescent and emit variously colored fluorescences, are colored or comprise inscription, or which possess diverse physical properties such as magnetism, electrical conductivity, thermo-magnetism etc. PA1 The presence of a watermark, that is, a controlled change in the density of the paper fibers throughout its thickness, this watermark showing a specific design which is only perfectly visible in transmitted light, i.e. by transparency. PA1 The presence of holograms, of moire patterns or other optical effects achieved using optically variable inks deposited at the sheet surface. PA1 The paper snap, that is a characteristic noise when the paper is quickly shaken. PA1 The presence of prints at the paper surface such as color inscriptions or drawings in complex forms that are difficult to reproduce. PA1 The printing relief obtained by intaglio printing. This process consists in engraving a plate, in spreading an ink on this plate and in pressing the sheet of paper on the plate. PA1 high printing quality, and thus PA1 high resistance to the effects of circulation, hence
As a rule, a securities document will not evince all such means because the cost of manufacturing the document increases with the number of identifiers and with their sophistication. However, the securities documents always comprise surface printing and therefore high-quality printing is required, both regarding the colors and the drawing, so as to make imitation difficult to counterfeiters. As a rule the public at large pays relatively little attention to the quality of the drawings and/or printing of the surface of a securities document, but trained bank personnel or merchants are quite sensitive to details of a drawing, its fineness and in general to its appearance, and accordingly are able to assess the authenticity of a document with the naked eye or with a magnifying glass.
The printing quality is taken from the fact that the line made by printing is perfectly clean, i.e., it lacks any smudges, and in the language of the man skilled in the art, it is free from "feathering". The result of such smudges is that the lines spread more than desired or that it is difficult to print lines that are very close without blurring. Consequently the central bank which imprints paper to obtain securities documents is forced to print in less than fine manner. It is therefore difficult to obtain microprints. Micro-printing is highly useful for security items because it is difficult to reproduce by photocopier which presently are unable to reproduce very fine detail. The printing quality also is characterized by the color density of the print.
Furthermore, the resolution of the human eye is adequate to distinguish between good print quality and print containing "feathering."
According to the prior art, the printing quality of the securities documents has been fairly mediocre and it is thus easy to counterfeit them using those color photocopiers that are able to reproduce almost exactly the color tones of an authentic document. Such counterfeiting would not be obvious to the eye, even of a trained person, since the printing quality of the authentic document is not sufficiently superior to that of the image from the photocopier.
Consequently, sheets for use in the manufacture of securities documents require high printing quality in order to obtain prints which are difficult to counterfeit.
Moreover, the quality and the beauty of the printing contribute in equal measure to the image of the document issuer. For example, in the case of banknotes, the printing quality contributes to the prestige of the country circulating them. This is another reason for care in printing such documents.
Regarding printing and writing papers, it is known to improve their printing quality by depositing a coat of a binder-pigment mixture on their surface. The purpose of this treatment is to level out the paper surface, which inherently is quite irregular. The pigment particles fill the interstices between fibers constituting the sheet.
There is a very wide choice of pigments and binders to make such a coat. As a rule the pigments are mineral fillers, but sometimes they are plastic pigments. The binders may be selected from starch, casein, animal paste, polyvinyl alcohol, natural and synthetic latex, etc.
For example, pigmented coats containing bentonites have been researched in the area of conventional printing-writing papers (TAPPI vol. 59, #12, December 76, New York, pp 76-80; R. L. Janes et al).
The French patent 999,579 filed by NCR and published in 1952 discloses a printing paper resistant to ink spreading which is prepared by coating with a composition of clay crystals and a binder. This binder may be starch, polyvinyl alcohol, animal paste, or casein.
Likewise, the French patent 999,625 also filed by NCR discloses a similar paper with the coating containing a zeolite substance instead of the clay. The binder may be starch alone or in combination with casein and/or latex.
French patent application A 2,288,186 discloses a printing-writing paper coated with a composition comprising a coating pigment, a binder and an amorphous mineral product obtained synthetically, which product improves the imprintability of the paper.
However, in the very special case of sheets or films used for securities documents, it is exceedingly rare that their surface is a pigmented coating. The printing applied to sheet-based securities documents must withstand all mechanical actions that might degrade it. Considering their exchange value, these securities documents circulate among individuals and move into various machinery, for instance forged-document sensors, automatic vending machines, etc. In the course of such frequent handling and circulation, the documents are subjected to treatment that may degrade them, for instance they are folded, crumpled, wetted, scratched, they undergo various frictions and even may be washed if accidentally put into a washing machine. In the case of gravure, when such a document is being printed, very high pressure (frequently in excess of 50 MPa) is applied to the engraved and inked printing plates, whereby the ink very deeply enters the sheet and the printing is thus protected. The paper must be well compressible. One of ordinary skill in the art knows that a coat on the sheet will form a sure barrier to the ink and will lower the penetration and adhesion of the ink and hence its life in circulation.
Obviously a long print life is desired regardless of the kind of printing employed, whether intaglio printing, offset printing or other. Naturally, a man in the art knows that putting a pigmented coat on the surface of a sheet for printing securities documents in order to improve its printability will degrade the strength and life of the print. By its inherent composition, this coat would not be very resistant to the circulation stresses to which the securities documents are subjected.
Furthermore, when in circulation, the securities documents undergo soiling. Resistance to the effects of circulation also implies resistance to soiling. Therefore the sheet must also possess surface properties repellant to water, grease and evincing low porosity.
As a rule, to improve resistance to the effects of circulation, especially of banknotes, a sheet of paper is impregnated prior to printing in a bath containing essentially one or more binders selected for their very high mechanical properties.
French patent application A 2,300,843 filed by Polysar Co. describes a coated sheet of paper prepared by a procedure in which a composition containing a flexible latex polymer and non-gelatinized starch granules of an average diameter less than the mean interstice distance between the surface fibers of the paper is spread on the surface of this paper, any excess of the coating composition being removed and the coated paper being left to dry. This sheet of paper can be used for banknotes or for other securities. The sheet is coated with such a composition to improve its surface condition and hence its printability. The filler is organic since it is starch, and its particle diameter is large. While this application does mention the problem of print-life when these documents are in circulation, it fails to suggest a solution and also is silent on a criterion for such a print life.